In previous work it was found that changes in length during contractures of cardiac muscle produce changes in activity, under certain conditions oscillations. It will be attempted to determine how the mechanisms underlying these changes come into play during normal contractions by recording the effect of various mechanical interventions during isotonic twiches. It is hoped that this work will furnish information on the effect of various conditions on the time course of the mechanical response of cardiac muscle. The studies will be extended to smooth and skeletal muscle. Preliminary experiments have shown that the level of sustained contraction of vascular smooth muscle is lowered strikingly by passive movements.